Don't Shatter My Image

Transcription

1 Don't Shatter My Image Name Physics - Reflection Lab This lab will locate images and relate the size of the angle at which the ray of light hits the plane mirror to the size of the angle at which the light is reflected. Materials: You will need a pink foam sheet, a plane mirror, mirror support, 2 straight pins, 2 colored pencils stuck in stoppers, a small black triangle, protractor, and ruler. Part A Tracing the Rays by Sighting 1. Place a piece of pink foam under the free piece of paper marked with positions P, A, B, and C. 2. Place the front silvered side of a mirror on the mirror line using a wooden mirror block. Place the wooden block sideways so the bottom of the mirror is flush with the paper. 3. Push a straight pin into the paper in front of the mirror at point "P" on the diagram. 4. Stick a pin at point "A". Sight across the head of pin "A", look into the mirror and move your head until pin "A" and the image of pin "P" are lined up. Mark a point near the mirror along the line between the mirror and "A". Call this point "a". 5. Remove the mirror and take the paper off of the pink foam sheet (I don t want huge pencil lines in the foam sheets). Draw a straight line from "A" through point "a" to the mirror line. Then draw a line from "P" to the mirror line where line "Aa" touches the mirror line. 6. At the point where line Aa touches the mirror, draw a line out from the mirror, perpendicular to the mirror (a normal line). Measure the two angles, one on each side of this perpendicular normal line. These are the angles of incidence and reflection. Record these angles in Data Table Set the mirror back on the paper and follow the same procedure for points "P and B" and "P and C". Record the pairs of angles here. Data Table 1 points angle of incidence angle of reflection P and A P and B P and C 8. How do the angles in each pair compare with each other?

2 Part B -Locating the Image of a Pencil 1. Grab 2 colored pencils stuck in the stoppers. Set one of the plane mirrors upright on a piece of plain paper, as shown in Figure A. Stand one pencil vertically in front of the mirror. Hold your eye steady at the height of the mirror. Locate the image of the pencil in the mirror. Place the second pencil where the image of the first appears to be (behind the mirror). If you have located the image correctly, the image of the first pencil and the second pencil itself will remain "together" as you move your head from side to side. Now the second pencil is located in the same place as the image of the first pencil. 2. How does the distance from the first pencil to the mirror compare with the distance from the mirror to the location of its image? 3. On the diagram below draw the path you think the light takes from the first straw to your eye. Be sure to include arrowheads on your path. mirror stopper o o eye

3 Mirror Line - Place Front Surface of Mirror Here * * * * A B C P

4 Name Do You Need All of the Mirror? The purpose of this activity is to find the minimum length of a plane mirror necessary to see a full view image of yourself. Materials: You will needed a full-length mirror, a transparency pen, and a ruler. 1. Stand 2 meters in front of a full-length mirror. Turn to the side so that you can see your heels in the mirror. 2. Have your lab partner mark the location on the mirror of the image of the top of your head and the bottom of your feet. Tell your partner where to mark the image as it appears to you, not how they look to your partner. 3. Measure the distance between the two marks. cm 4. Stand a few meters further from the mirror. As you move farther from the mirror, the marks your partner would make to mark the top and bottom of you: (circle one) a. get much farther apart b. stay pretty much the same c. get much closer together 5. Measure your actual height. cm 6. Diagram the reflecting light from your head and feet to your eyes. In other words, trace the ray paths. 7. How did the length of your image on the mirror compare with your actual height? What fraction of your height does the length of your image represent? 8. Clean the mirror off.

5 The Kaleidoscope The purpose of this activity is to apply the law of reflection to a mirror system, which causes multiple reflections. Materials: You will need 2 plane mirrors, mirror supports, protractor, pin. 1. Using the cardboard wedge, place the two mirrors at an angle of 72 with one edge touching together, as in the diagram. Be sure the mirrored surfaces are facing forward and hold the mirror together at the back so there is no "gap" where the mirrors meet. 2. Use the circled 72 as your OBJECT. 3. Count the number of images you can see resulting from this system. Record your data in the table below. Angle Between the Mirrors Number of Images Seen Change the angle of the mirrors using the other wedges and count the images. Complete the table. 5. Use your math skills to determine the relationship between the angle and the number of images. What is the formula for calculating the number of images? 6. Use the formula you figured out to predict how many images you would see if the mirror were separated by an angle of only 5. Show your work.

6 Retro-reflectors The purpose of this activity is to study the reflection from mirrors mounted at right angles. Materials: You will need a bicycle reflector. 1. Consider the reflectors you have seen such as on bicycles, running gear, and sign posts along the side of the interstate. They consist of molded plastic with facets (faces) that make angles to one another. Observe the bicycle reflectors in close detail with a magnifying glass. Can you see the angle between the faces? Can you estimate the magnitude of the angle? 2. At night, these reflectors are most likely to be illuminated by the headlights of a car. Use a ruler and a protractor to draw two lines at a 90 angle representing the relationship between two faces of a bicycle reflector. Draw an incident ray at some angle hitting one of the lines. Use the ruler, protractor, and the law of reflection to trace the rays as they reflect twice off of the two mirror faces (lines). These examples demonstrate the purpose of the multi-faced reflectors on a larger scale. The small example shown is for demonstration only, and may not be perfect a 90 angle.

7 3. Look carefully at the facets of the bike reflector again. How are these facets arranged with respect to each other? (hint: it's related to this lab) 4. What do you predict about the direction an incident ray would be reflected from this reflector? 5. Why are these reflectors used rather than a flat reflecting surface? What is the purpose of the reflectors? 6. The Apollo astronauts left 7 reflectors on the moon. We use them to reflect laser light and help us determine exactly how far away the moon is. Would you expect these reflectors to be plane mirrors or retro-reflectors? Explain your choice.

8 Faces In this lab you will use your reflection to draw a picture of your own face. Materials: You will need a transparency pen, transparency sheet and a large plane mirror. 1. Place the large mirror on the table and place the transparency sheet over it. Position yourself comfortably over the mirror. 2. Tape the transparency sheet on the mirror where the image of your face appears. 3. Use the transparency pen to carefully trace the outline of your face. Take your time and draw carefully. If you make a mistake, use a damp towel to erase that part. 4. Back up a couple of feet. Does your image still fit into the drawing? Explain your observation. Use ray diagrams to help you explain. 5. How does the height for your drawing compare with the height of your head? (circle one) 4X 2X 1X 3/4X 1/2X 1/4X 6. How does the width of your drawing compare with the width of your head? (circle one) 4X 2X 1X 3/4X 1/2X 1/4X 7. How does the area of your drawing compare with the area of your face? (circle one) 4X 2X 1X 3/4X 1/2X 1/4X 8. Sign your name and give the completed picture to your teacher so that it can be photocopied (made permanent). These drawing make excellent (cheap) gifts for your parents.

9 Name Objects in Mirror Are Closer (or Farther) Than They Appear In this lab you will investigate images formed by curved (concave and convex) mirrors. Various sizes of convex and concave mirrors, plane mirrors. Part 1 - Small concave mirrors 1. Hold the plane mirror at arm s length. Observe your image. 2. Now hold the small concave mirror at arm s length. Compare this image to the one with the plane mirror. Compared to the image in the plane mirror, the image in the concave mirror is: (smaller / larger / same size) (upright / inverted) (closer / farther away) 3. Now look at the iris of your eye by holding the concave mirror very close. Compare this image with the image in a plane mirror held at the same distance. Compared to the image in the plane mirror, the image in the concave mirror is: (smaller / larger / same size) (upright / inverted) (closer / farther away) 4. While looking at the iris of one eye, open and close the other and watch the pupil change size. 5. After focusing on your iris, try to focus on your eyelashes. Do you have to move the mirror toward your eye or away from your eye? (toward / away from) Part 2 - Convex Mirrors 6. Hold the convex side of the metal mirror at arm s length. Observe your image. Compared to the image in the plane mirror, the image in the convex mirror is: (smaller / larger / same size) (upright / inverted) (closer / farther away) 7. Why are mirror like these used in stores and as rear view mirror in cars? What advantage do they provide over a plane mirror of the same size? 8. Why must auto manufacturers print a warning on driver-side rear view mirrors? Why does it matter if the "objects in the mirror are closer than they appear"? 9. Look in the back (convex) side of the spoon. Apart from its size, how does the image in the spoon differ from the image in the convex spherical mirror?